1. Field of the Invention
The invention generally relates to vision diagnostics and, more particularly, to a method for providing improved objective manifest refraction values, an associated method for prescribing a vision correction, and apparatus associated therewith.
2. Description of Related Art
A phoropter is a fundamental optometric diagnostic instrument for vision measurement and evaluation for obtaining a manifest refraction; i.e., defocus and astigmatism (often referred to as xe2x80x9clower-orderxe2x80x9d aberrations) in an undilated eye. It essentially is a device with a large set of lenses on dials. The device is positioned for a patient to look through and give visual acuity feedback to the practitioner when a particular dialed lens is presented in front of the patient""s eye. This method of manifest refractometry provides defocus and astigmatism information to the practitioner typically in order to prescribe vision correcting lenses for the patient. The subjective nature of the phoropter measurement process, from the patient""s perspective, is itself a disadvantage of this form of refractometry. Practitioner error can also be problematic, especially when adequate practitioner training may be lacking as it often is in many underdeveloped areas of the world.
An autorefractor is a device that provides an objective diagnostic measurement of a patient""s refraction. Although patient subjectivity has been removed from the measurement process, there are other disadvantages associated with autorefractors. First, they are expensive instruments. Second, autorefractor measurements are typically inaccurate, compared to a patient""s subjective refraction. There are reports of measurement errors in up to 20% of the population measured in this way. In fact, up to a 2 diopter (D) difference between the objective manifest refraction and subjective manifest refraction has been observed on an individual basis.
A wavefront sensor is a device that measures optical errors in terms of wavefront aberrations. The measured aberrations typically include monochromatic wavefront defects such as spherical aberration, coma, trilateral astigmatism and others, usually referred to as higher-order aberrations. Although wavefront sensing has been used for some time in astronomical and defense applications, the modification, use, and development of this technology in ophthalmology is relatively recent. Moreover, wavefront sensor data is not naturally indicative of manifest refraction. Yet, as vision correction technology advances, wavefront sensing instrumentation will, by necessity, consume office space and resources. Broadening the usefulness of such a tool will justify the costs associated with these instruments.
Based on the foregoing, the inventors have recognized the desirability of being able to accurately predict a manifest refraction based upon objective manifest refraction data, and do it efficiently. Thus, obtaining better measurements with less equipment and less expensive equipment is highly advantageous. The invention also provides an improvement in the ability to specify and prescribe vision correction, including lenses and refractive surgical treatment. Apparatus associated with the invention is further provided. These and other advantages and objects of the invention are described in detail below and with reference to the appended claims.
The invention, as one of its general objects, is directed to providing an accurate manifest refraction value (referred to herein as xe2x80x9cpredicted phoropter refractionxe2x80x9d or xe2x80x9cPPRxe2x80x9d) from objective measurement data, particularly a wavefront measurement.
In an embodiment of the invention, a method for providing an improved objective manifest refraction includes the steps of objectively obtaining diagnostic measurement data of a patient""s eye that is indicative of at least fourth-order Zernike wavefront aberrations or their equivalents, and fitting a second-order only Zernike polynomial to the wavefront data to determine a simplified surface represented by the wavefront information obtained in the preceding step; and for calculating a manifest refraction value from the second-order surface calculation data that accurately corresponds to a subjective manifest refraction value. In various aspects, the wavefront measurement data will preferably include at least fifth and higher-order terms, up to seventh-order terms, and up to tenth-order terms. In an aspect of this embodiment, fitting the second-order Zernike polynomials to the higher-order wavefront data uses a least squares method. The objectively calculated refraction according to the invention (i.e., the predicted phoropter refraction, or PPR) is an accurate rendering of a patient""s actual subjective refraction. An accurate PPR is one that is preferably within 0.75D to 0.5D of the patient""s subjective refraction; more preferably within 0.5D to 0.25D; and most preferably less than a 0.25D difference from the patient""s actual subjective refraction.
A Zernike expansion is a preferred way to describe the aberrations of an optical system. A Seidel aberration model is one of several alternative descriptions of optical aberrations. For more detailed information on this topic the reader is referred to Born and Wolf, Principles of Optics (Pergamon, N.Y., 1975), and to Geary, Introduction to Wavefront Sensors, SPIE Optical Engineering Press (1995), both of which are incorporated herein by reference in their entirety to the extent allowed by applicable patent rules and laws.
An advantageous offered by the invention is the ability for accurately specifying and prescribing a vision correction for lenses such as spectacle, intra-ocular, and contact lenses, for example, as well as for refractive surgical modification of the cornea, such as LASIK, LASEK, or PRK.
In another embodiment of the invention, a display of an optical diagnostic measurement of a patient""s eye, which is typically associated with the measurement apparatus and procedure includes an image representation of second-order and lower aberrations; and an image representation of all measured wavefront aberrations including low-order and higher-order aberrations. In an alternative aspect of this embodiment, the display includes an image representation of an astigmatic wavefront measurement only; and an image representation of third-order and higher wavefront aberrations. Preferably, the displays in both aspects of the embodiment described above will include indicia of the PPR. Preferably, the PPR will be provided for a patient""s pupil size of approximately 3 to 4 mm in diameter, and more preferably at a pupil diameter of 3.5 mm. The PPR indicia can optionally be made available for display over a full range of pupil diameters through actual measurement or by appropriate calculations, as understood to those skilled in the art, and incorporated in the hardware or software involved. Moreover, the preferred display will show a vision quality indicator (referred to as a vision metric) such as a Point Spread Function or a Strehl ratio, for example.